Methane adsorption amount and kinetics in coal with pre-adsorbed water

Coalbed methane (CBM) has emerged as a significant unconventional resource, and understanding the interactions between adsorbed water and methane in coal is crucial for assessing coalbed methane reserves and productivity. To elucidate how pre-adsorbed water influences methane adsorption, we conducted high-pressure methane adsorption experiments on both dry and wet coals with various maturities. By employing a combination of theoretical modeling and experimental analyses, we have identified that adsorbed water could markedly decrease the amount of methane sorption, and changes in water coverage are responsible for the observed negative correlation between water coverage and methane pressure for these samples. Further investigations involving wettability measurements and low-pressure N₂ adsorption demonstrate that adsorbed water exhibits weaker interaction with highly mature coal compared to the low-maturity coal. Our analyses suggest that adsorbed water can decrease methane adsorption quantity in coal by up to 60%. Moreover, the adsorption and mass transfer rates of methane can be reduced by up to 90% and 80%, respectively, due to the presence of adsorbed water. Additionally, our results indicate that methane can displace adsorbed water in highly mature coal significantly, decreasing water coverage from 63% to 22% with increasing methane pressure.